[fix] Hexbin layer color aggregation incorrect on load

[igorDykhta]

PR Description

• Fixes loading cycle layer validation in vis state mergers by moving validateSavedVisualChannels() to be done after newLayer.validateVisConfig() and newLayer.updateLayerConfig().

  Specifically, I found that when loading a Hexbin layer with a saved config color aggregation mode of count unique, the visual channel color aggregation would be incorrectly validated and set with options that do not work together, first without enough of the saved config included to help it understand which field type, color scale, and aggregation mode should be used, despite the saved config information being correct from the previous session. This change does the visual channel validation after all saved config information is mixed into the newLayer.

• Improves aggregation dropdown UI to use more human-friendly options to display to the user.

• Bonus: Improves displayed instruction text above the layer configurator aggregation type selector UI, so that it uses the selected field's displayName instead of raw name.